In a radio communications system, it can be of importance to be able to determine the time at which signals are transmitted by radio transmitter of the system.
The possibility to determine the timing of transmissions of different radio transmitters can be employed e.g. advantageously to geographically localize a mobile station in a radio communications system. To this end, e.g. first the timing difference between at least two pairs of radio transmitters is determined. The timing difference can be determined based on the measurements of at least one radio receiver positioned at a known location. The mobile station moreover measures the difference in the reception time of signals transmitted by the same pairs of radio transmitters.
The location of the mobile station can then be calculated based on the time differences measured at the mobile station and the known timing difference of the at least two pairs of radio transmitters.
For illustration, an example of such a localization method will now be described in more detail with reference to FIG. 1.
FIG. 1 schematically shows a GSM (Global System for Mobile communications) radio communications system with three GSM base transceiver stations BTS 1–3. The base transceiver stations BTS 1–3 constitute a network of radio transmitters. Further, two radio receivers LMU1,LMU2 and a mobile station MS are shown. In GSM Location Services standards, such receiving units are called Location Measurement Units (LMU). In the depicted system, the current location of the mobile station MS is to be determined.
A network of radio transmitters like the depicted base transceiver stations BTS 1–3 has some timing system, which is based for example on respective internal clocks or on a clock of an element located higher in the network hierarchy. Each radio transmitter transmits radio signals according to the clock signal with which it is provided and some set of rules. The time differences between the used clocks determines how much earlier or later one radio transmitter sends a signal than a respective other radio transmitter.
Since GSM is a TDMA (Time Division Multiple Access) system, the depicted GSM base transceiver stations BTS 1–3 transmit radio bursts in time slots using basically the same time slot structure. To this end, each GSM base transceiver stations gets a hierarchical clock signal from the network, e.g. through PCM (Pulse Code Modulation) lines. In theory two base transceiver stations, for example BTS 1 and BTS 2 of FIG. 1, should therefore send TDMA bursts exactly at the same moment. However, this is not the normal case. In practice, timing differences still exist. The timing difference between two base transceiver stations is also referred to as Real Timing Difference (RTD). If base transceiver station BTS 1 sends a burst at the time t1 and base transceiver station BTS 2 sends a burst at the time t2, the Real Timing Difference between them is RTD=t1−t2 or RTD=t2−t1, depending on the definition of the RTD. Only if the network is really synchronized, the Real Timing Difference is zero.
A possibility for determining the Real Timing Difference between different radio transmitters is described e.g. in PCT/EP97/02400. For the method proposed in this document, radio receivers have to be situated at fixed positions around the radio transmitters. In FIG. 1, this is given by the measurement units LMU1,2 which are positioned at known locations close to the base transceiver stations BTS 1–3.
The first measurement unit LMU1 then receives a radio signal from the first base transceiver station BTS 1 and from the second base transceiver station BTS 2, and determines the time interval between these two receptions: The time interval is the so-called Observed Time Difference (OTD). In case a burst from the first base transceiver station BTS 1 is received at a time t3 and a burst from the second base transceiver station BTS 2 is received at a time t4, the OTD for this pair of base transceiver stations is OTD=t4−t3 or OTD=t3−t4.
It is to be noted that in GSM, the measurement units can be similar to a mobile station. In fact in GSM, OTD is already specified as a function for mobile stations in order to support the so called pseudo-synchronous handover. OTD is in this case the time interval, as observed by the mobile station, between the reception of two bursts from two different base transceiver stations. For OTD measurements in mobile stations, in practice one base transceiver stations is the serving base transceiver station, and the other one is one of the neighbor base transceiver station.
The OTD determined at measurement unit LMU1 is composed of the RTD between the first base transceiver station BTS 1 and the second base transceiver station BTS 2, and of a Geometrical Time Difference GTD:OTD=RTD+GTD  (1)
More specifically, the Geometrical Time Difference is the time difference in transmission between two signals from two different transmitters (BTS) to a single receiver (LMU,MS) due to geometry. If the distance of the first base transceiver station BTS 1 to the first measurement unit LMU1 is d1 and the distance of the second base transceiver station BTS 2 to the first measurement unit LMU1 is d2, as indicated in FIG. 1, then the Geometrical Time Difference is GTD=(d1−d2)/c, where c is the speed of radio waves. Thus, since the positions of the base transceiver stations BTS 1,2 and the first measurement unit LMU1 are known, GTD values can be calculated, OTD values be measured, and RTD values be obtained.
The same measurements are carried out by the second measurement unit LMU2 for obtaining the timing difference RTD of base transceiver stations BTS 1 and BTS 3. In this case, the GTD is calculated based on a distance of the first base transceiver station BTS 1 to the second measurement unit LMU2 of d3, and on a distance of the third base transceiver station BTS 3 to the second measurement unit LMU2 of d4, as indicated in FIG. 1. Alternatively, also the RTD between base transceiver stations BTS 1 and 3 could have been determined based on measurements by the first measurement unit LMU1, in case this measurement unit is also located close enough to base transceiver stations BTS 3 for receiving signals.
The thus known Real Timing Difference between at least two pairs of base transceiver stations BTS 1,2 and BTS 1,3 can then be exploited for determining the geographical location of the mobile station MS.
An example for an E-OTD (Enhanced OTD) location method for GSM has been standardized in GSM 03.71 “Location Services (LCS) Stage 2”, which contains also a general description of E-OTD. The E-OTD location method described in the standard is based as well on the above mentioned equation (1), only in this case, the RTD is known, while the GTD is to be determined.
If the GSM mobile station MS performs OTD measurements for signals received from pairs of base transceiver stations BTS 1,2 and BTS 1,3 respectively, and corresponding RTD values are known for these pairs, e.g. measured by special RTD receivers as described above, then the Geometrical Time Difference can be calculated for the mobile station for each pair of base transceiver stations BTS 1,2 and BTS 1,3 based on the above mentioned equation (1). The respective Geometrical Time Difference and the information of the position of the corresponding pair of base transceiver stations BTS 1,2, BTS 1,3 define a hyperbola H1, H2 on which the mobile station is located. When at least two hyperbolas H1, H2 are defined, the estimate of the location of the mobile can be found at their intersection as shown in FIG. 1.
One possible location system is such that mobile stations perform OTD measurements, and report the results to the network, where location calculation is done based on the known RTD.
If receivers, like Local Measurement Units, have absolute time available, they can report Absolute Time (AT) values and/or Absolute Time Difference (ATD) values as reception times of received signals to the network. Absolute time can be made available to receivers e.g. based on the Global Positioning System (GPS). The AT value is a time stamp based on the absolute time clock for the reception of the signal from a transmitter like a base transceiver station. ATD is the same as RTD with the exception that also at least one AT of the received signals of the pair of transmitters is known.
A problem connected generally to RTD and AT/ATD measurements are Non Line Of Sight (NLOS) conditions. If there is a NLOS situation between a radio transmitter and a radio receiver, the signal is reflected and thus its propagation path contains extra length. The estimate of the geographic influence on the received signals based on known transmitter and receiver coordinates, e.g. using GTD, is consequently wrong, and the AT/ATD or RTD estimate includes an extra error. Similar problems are encountered with multipath conditions, i.e. when a Line of Sight (LOS) component is present between transmitter and receiver, but also strong reflected components, which might make a detection of the LOS component difficult.